Balloon catheters are well-known devices in which the catheter carries an inflatable balloon to occlude and seal a body space, to expand a blood vessel through pressurized inflation of the balloon, or for any other desired purpose which may typically but not necessarily be a therapeutic purpose in the medical field. In the case of dilatation balloon catheters for angioplasty, for example a PTCA procedure, the catheter balloon is commonly made out of a thin, strong material which is of relatively low resilience. Examples of materials from which a catheter balloon may be made include biaxially oriented polyethylene terephthalate (PET) copolymers, a polyamide material such as nylon or polyether-block-amide copolymers to mention only a few. These materials are typically strong, flexible materials and have the advantage that they are flexible but inelastic so that they can expand outwardly to a predetermined diameter, and then cease further expansion at normal pressures, to avoid damage to the artery wall by over expansion.
The fact that balloon angioplasty requires extremely thin walled material, high strength (i.e. high tensile), relatively inelastic material of predictable inflation properties, leads to a common problem which is that the balloons can be easily punctured through abrasion or the like, even though they have a high tensile strength. Pin holes and ruptures are a fairly common problem when such catheter balloons are used in contact with rough surfaces. Also, tiny flaws in the mold of such balloons can create weak spots, since the balloons are so thin-walled. However, it is impractical to increase the wall thickness of these balloons because then they become too stiff, with high flexural moduli, with the result that such balloons do not collapse properly on deflation to facilitate easy withdrawal from the vascular system of a patient.
Further, thin walls are necessary because the balloon's wall and waist thicknesses limit the minimum diameter of the distal end of the catheter and therefore determine the limits on vessel size treatable by the method and the ease of passage of the catheter through the vascular system. High strength is necessary because the balloon is used to push open a stenosis and so the thin wall must not burst under the high internal pressures necessary to accomplish this task. The balloon must have some elasticity so that the inflated diameter can be controlled, so as to allow the surgeon to vary the balloon's diameter as required to treat individual lesions, but that elasticity must be relatively low so that the diameter is easily controllable. Small variations in pressure must not cause wide variation in diameter.
The high strength biaxially oriented polyethylene terephthalate (PET) or polyamides such as nylon or the polyamide copolymers tend to be less elastic, and have less resilience. Balloon catheters may be made of more elastic materials such as polyolefins or polyolefin copolymers, but typically, in order to achieve the high tensile strength, the balloon walls must be made thicker.
Coatings may be used to modify the surface of the balloon to improve abrasion and puncture resistance, but such coatings typically increase the wall thickness, and decrease the flexibility and thus increase the stiffness and flexural moduli of the balloon. This increased stiffness and increased flexural moduli can result in balloons which do not collapse properly on deflation to facilitate easy withdrawal from the vascular system of a patient.
Coatings may be used for other purposes such as increasing lubricity as well and for delivering therapeutic agent(s), for example. Typically, such coatings are provided by dipping, brushing, painting, and so forth.
U.S. Pat. No. 6,287,277 describes balloons formed by a vacuum deposition process.
U.S. Pat. No. 5,451,428 describes a plasma polymerizing an intermediate layer onto a medical device which is then used to react with a biocompatible coating to provide optimum adhesion of the biological coating.
There remains a need for materials and methods which can provide improved balloon catheters which are thin walled, yet have a durable, abrasion and tear resistant surface thereby improving the resistance to pinhole formation, and yet are relatively flexible, yet inelastic to allow the balloons to expand outwardly to a predetermined diameter, and then cease further expansion at normal pressures, to avoid damage to the artery wall by overexpansion.